Essential Tools
New TongWenTang
New TongWenTang is a broswer extension that swaps simplified characters for traditional characters or vice versa. You can make is so that it forces the change on a website all the time, or you can just toggle between.
Zhongwen
Zhongwen is a browser extension that translates characters by hovering over them. Very useful tool that supports both Simplified and Traditional characters - lets you toggle Zhuyin characters on. I made a modified verson that makes it so Pinyin characters are the same colour as the background - so you only get the Hanzi and the Zhuyin.
Get my modified version for Firefox
Zhuyin
Zhuyin is a browser extension by Phoebe Tai, originally for Chrome only - I have made a Firefox port of it. Zhuyin swaps out the font pack so that Zhuyin Ruby characters are added to the Traditional Chinese characters. It only works on Traditional characters.